1. Field of the Invention
The present invention relates to gas turbine engines, and more particularly to gas turbine engines utilizing low calorific value fuels.
2. Description of Related Art
Gasification of coal, biomass, and other fuels produces fuel gas that can be used for power production. Fuel gas derived from gasification or other such processes is commonly referred to as low calorific value (LCV) fuel because it typically has significantly lower heating values compared to more traditional fuels. Whereas natural gas typically has a heating value of about 1,000 BTU/Ft3, LCV gas can have a heating value on the order of only about 130 BTU/Ft3 and less. LCV gas can be used with or as a replacement for more traditional fuels in applications including internal combustion engines, furnaces, boilers, and the like. In addition to environmental concerns, fluctuating fuel costs and availability drive a growing interest in use of LCV fuels where more traditional fuels, such as natural gas, are typically used.
While there is growing interest in LCV fuels, the low heating value of LCV fuel creates obstacles to its more widespread use. Thus there is an ongoing need for improved LCV fuel combustion systems. For example, the use of LCV fuel in an existing, conventional gas turbine engine requires special considerations regarding the fuel injection system. Flammability of LCV fuel gas can be unknown due to variables in the gasification process, so there is typically an unpredictable flameout limit when lowering fuel flow to operate at reduced power. Due to the relatively low heating value, LCV fuel can require 10 to 12 times the volumetric flow rate of natural gas for which the original engine was designed, which can give rise to capacity complications for traditional combustion systems. Typical gasification systems produce LCV fuel through high-temperature processes, and LCV fuel is often supplied directly from the gasification system. The LCV fuel temperature can be significantly hotter than in conventional fuel systems, which can give rise to further thermal management concerns. Additionally, due to the low calorific value, the fuel can present difficulties in terms of start up and flame stabilization.
Some solutions to these challenges have been proposed, such as using large numbers of small injectors, and allowing for mixing traditional fuel in with LCV fuel. However, the high flow rates needed to provide an adequate supply of LCV fuel lead to significant pressure drop, which is exacerbated by using large numbers of small injectors. High pressure drop can severely impact overall thermal efficiency for gas turbine engines, for example. Start up and flame stabilization challenges persist in typical LCV fuel injection systems.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for combustion systems and methods that allow for improved start up, flame stability, and fuel staging. There also remains a need in the art for such systems and methods that are easy to make and use. The present invention provides a solution for these problems.